Satellite communications systems and methods are widely used for wireless communications of voice and/or data. Satellite communications systems and methods generally employ at least one space-based component, such as one or more satellites that are configured to wirelessly communicate with a plurality of wireless terminals.
A satellite communications system or method may utilize a single antenna beam (antenna pattern) covering an entire service area served by the system. Alternatively, in cellular satellite communications systems and methods, multiple antenna beams/cells (antenna patterns) are provided, each of which can serve substantially distinct geographical areas in an overall service area, to collectively serve an overall satellite service area. Thus, a cellular architecture similar to that used in conventional terrestrial cellular radiotelephone systems and methods can be implemented in cellular satellite-based systems and methods. The satellite typically communicates with wireless terminals over a bidirectional communications pathway, with communication signals being communicated from the satellite to the wireless terminal over a downlink or forward link, and from the wireless terminal to the satellite over an uplink or return link. The downlink (forward link) from a satellite to a radioterminal and/or the uplink (return link) from a radioterminal to a satellite may be referred to as service link(s).
The overall design and operation of cellular satellite systems and methods are well known to those having skill in the art, and need not be described further herein. Moreover, as used herein, the term “wireless terminal” includes cellular and/or satellite radiotelephones with or without a multi-line display; Personal Communications System (PCS) terminals that may combine a radiotelephone with data processing, facsimile and/or data communications capabilities; Personal Digital Assistants (PDA) that can include a radio frequency transceiver and/or a pager, Internet/Intranet access, Web browser, organizer, calendar and/or a global positioning system (GPS) receiver; and/or conventional laptop and/or palmtop computers or other appliances, which include a radio frequency transceiver, for wireless voice and/or data communications. A wireless terminal also may be referred to herein as “terminal,” “radiotelephone,” “user device” and/or “radioterminal”. As used herein, the term “radioterminal,” or any of its equivalent terms listed above, also includes any other radiating user device/equipment/source that may have time-varying or fixed geographic coordinates, and may be portable, transportable, installed in a vehicle (aeronautical, maritime, or land-based), or situated and/or configured to operate locally and/or in a distributed fashion at any other location(s) on earth and/or in space.
Cellular satellite communications systems and methods may deploy hundreds of cells (antenna patterns or spot beams), over their satellite footprint corresponding to a service area. It will be understood that large numbers of cells may be generally desirable, since the frequency reuse and the capacity of a cellular satellite communications system or method may both increase in direct proportion to the number of cells. Moreover, for a given satellite footprint or service area, increasing the number of cells may also provide a higher gain per cell, which can increase the link robustness and improve the quality of service.
The uplink and downlink communications between the wireless terminals and the satellite may utilize one or more air interfaces, including proprietary air interfaces and/or conventional terrestrial cellular interfaces, such as Time Division Multiple Access (TDMA) and/or Code Division Multiple Access (CDMA) air interfaces. A single air interface may be used throughout the cellular satellite system. Alternatively, multiple air interfaces may be used by a satellite communications system. See, for example, U.S. Pat. No. 6,052,560, issued Apr. 18, 2000, entitled Satellite System Utilizing a Plurality of Air Interface Standards and Method Employing the Same, by the present inventor Karabinis. In general, regardless of the air interface or interfaces that are used, each satellite cell generally uses at least one carrier/channel to provide service. Thus, the return service link and the forward service link each use one or more carriers/channels to provide service.
The above description has focused on communications between the satellite and the wireless terminals. However, cellular satellite communications systems and methods also generally employ a bidirectional feeder link for communications between a satellite gateway and the satellite. The bidirectional feeder link includes a forward feeder link from the gateway to the satellite and a return feeder link from the satellite to the gateway. The forward feeder link and the return feeder link each use one or more carriers/channels.
As is well known to those having skill in the art, the number of satellite cells and the air interface or interfaces that are used may impact the bandwidth that is used in the feeder link from the satellite gateway to the satellite and from the satellite to the satellite gateway. For example, if a cellular satellite system and method deploys 400 service link cells and uses a narrowband CDMA air interface to provide communications between the satellite and the wireless terminals, each CDMA carrier that is transported from the satellite gateway to the satellite may consume 1.25 MHz of feeder link spectrum. Assuming that traffic is such that only one carrier per cell is used, then 400×1.25 MHz or 500 MHz of forward feeder link bandwidth may be used. Moreover, if certain cells use more than one carrier and/or a Wideband CDMA (W-CDMA) air interface standard is used, the feeder link bandwidth may increase further.
U.S. Pat. No. 6,317,583 to Wolcott et al. describes a telecommunications satellite channelizer for mapping radio frequency (RF) signals between feeder links and mobile link beams based on a predefined frequency plan. The mobile link beams define a coverage area of a satellite. Each feeder link and mobile link beam comprises a plurality of feeder subbands and mobile subbands are grouped to form feeder link channels and mobile link channels. The channelizer includes at least one feeder lead carrying a feeder link signal associated with a ground station. A feeder link distribution network is connected to the feeder leads and maps RF signals in the feeder links onto a plurality of distribution leads as divided feeder signals. Channel multiplexers are connected to the distribution leads. Each channel multiplexer includes a set of band pass filters, each of which passes RF signals in a subset of feeder subbands corresponding to a single feeder channel in order to map a mobile link channel and a feeder link channel onto one another based on a predefined frequency plan. The channelizer groups or multiplexes signals from a plurality of feeders into each beam. Fixed local oscillator up converters shift each composite mobile channel to a common band allocated to all beams. The frequency plan is defined such that beam handovers and ground station handovers may be performed without a need for at least one of switching, retuning and resynchronization of the telecommunications satellite and the mobile terminal. See the Wolcott et al. Abstract.
U.S. Pat. No. 5,903,549 to von der Embse et al. describes a method permitting beam forming at a ground station for providing a coherent and stable uplink signal to a satellite system employing multiple spot beams by combining orthogonal synchronous code division multiplex codes and pseudo-random spreading codes with an information signal to implement a satellite feeder uplink.
Finally, published U.S. Patent Application US 2003/0224785 A1 to the present inventor Karabinis describes that information content is nonidentically mapped between service link carriers and feeder link carriers at a cellular satellite. A reduced number of satellite feeder link carriers compared to the number of satellite service link carriers and/or a reduced total bandwidth of the satellite feeder link carriers compared to the satellite service link carriers thereby may be obtained.